# Arnau Rios Huguet

### Ramón y Cajal Fellow, University of Barcelona

## Who am I?

I am a theoretical nuclear physicist studying quantum many-body systems at the HadNucAtUB group in at the University of Barcelona and the Institute of Cosmos Sciences. I also hold a visiting professorship at the University of Surrey.

Students or post-docs interested in carrying out research projects on theoretical nuclear and/or astronuclear physics and looking for funding opportunities, please contact me (see below for details). You can find more about me in my twitter feed @riosarnau.

# News

## MACHINE LEARNING 1D FERMION SYSTEMS

11 Apr 2023

The main results of James Keeble's PhD thesis have been submitted for publication, and are available as a preprint in this arxiv link 2304.04725! We find that a single neural network architecture can describe the properties of 1D "spinless" (or fully-polarized) fermions interacting with a finite-range, gaussian potential. We benchmark our results with different quantum many-body methods, and we find that the NQS is variationally better than some in different regimes. The network predicts the appearance of two distinct physical phases: a bosonization transition in the attractive regime, and a crystalline phase in the repulsive one.

## QUANTUM COMPUTING NUCLEI

8 Feb 2023

In a new preprint (arxiv link 2302.03641) with Antonio Márquez Romero (Margarita Salas Fellow here at UB) and colleagues at the Barcelona Supercomputer Center and Qiliminjaro, we study the performance of a variational quantum eigensolver (ADAPT) to find the ground state properties of several isotopes. We develop a strategy to generate the necessary quantum circuits to measure the energy and apply it to a variety of nuclei in the p, sd and pf shells. It is exciting times for quantum computing!

Update, August 2023: published in Scientific Reports, doi:10.1038/s41598-023-39263-7.

## PAIRING CRITICAL TEMPERATURE OF NEUTRON MATTER

2 May 2022

A recent preprint (arxiv link 2202.07501) with Mehdi Drissi (currently at TRIUMF) has just been published at the European Journal of Physics A (link here). In this paper, we took a critical look at the pairing critical temperature in neutron matter. We show that some approximations for superfluidity predict ratios of the critical temperature to the zero-temperature gap that are different from the BCS prediction. The ratios are relatively independent of the nuclear force that is employed, and the values agree with predictions for ultracold gases. This may have implications for neutron-star cooling simulations!

## WHAT IS THE SATURATION ENERGY OF NUCLEAR MATTER?

In a just-published paper in Phys Rev C here (arxiv link 2001.07231) with colleagues at Washington University in St Louis and Argonne National Laboratory, we have critically looked at the saturation energy of infinite nuclear matter. Using 3 different theoretical methods, we conclude that it is possible for this energy to deviate from the canonical value of E=-16 MeV. Finite nuclei fits of the dispersive optical model suggests it could be as low as E=-13 MeV, which could have important consequences for a variety of nuclear phenomena.

## GREEN'S FUNCTIONS FOR INFINITE NUCLEAR MATTER

I have published a short review on self-consistent Green's functions methods and their potential in the study of infinite matter here (arxiv link 2006.10610). I tried to highlight the advantages of this approach, which yields a broad range of information in a single simulations. We can predict the equation of state (of interest for neutron stars), but also get the in-medium spectral strength of nucleons (see contour plot here!).

## MACHINE LEARNING THE DEUTERON

My PhD student James Keeble has solved a nuclear physics problem using machine learning techniques! Using the ready-made PyTorch suite, he has been able to find the ground state of the deuteron (the only bound state of a neutron and a proton) with extreme accuracy using a very simple artificial neural network as a wavefunction. We think this paves the way for new techniques of nuclear many-body based on machine learning. Find out more in our preprint here 1911.13092 and watch this space for further updates!

## HELLMANN-FEYNMAN THEOREM

The Hellmann-Feynman theorem is a key result in quantum mechanics that is used across disciplines, from quantum chemistry to particle physics. In my latest preprint at arxiv:1909.10298 (now published in the American Journal of Physics), I worked with collaborators at Barcelona and Catania and looked at the extension of the theorem to finite temperatures. We applies the theorem to 3 very different quantum mechanical systems, hoping that this analysis can potentially be used in quantum mechanics or statistical mechanics teaching.

## AB INITIO WORKSHOP

From 24 to 26 July 2019, I co-organised with colleagues at Surrey a workshop on Ab Initio Nuclear Theory. Researchers from around the world came to Guildford to discuss the most recent breakthroughs in nuclear theory, which have allowed for a real explosion of first-principles predictions. We asked ourselves the question what next in ab initio theory? and had fun trying to answer it!

## SURREY NUCLIDE CHARTS

My nuclear experimental colleagues are world leaders in their field. They customarily investigate unexplained nuclear phenomena in experimental facilities across the world. More importantly, they work at the discovery frontier and have in fact discover a number (>150) isotopes on their own experiments. Click on the figure on your left to get a full nuclide chart where Surrey-discovered isotopes are highlighted in yellow! Impressive, isn't it?

The figure has been created with gnuplot, using the nuclear data in Nubase2016 and the historical data provided by the Discovery of the Nuclides project.

# Contact details

Arnau Rios Huguet

Office V520 (Old Building, Floor 5),

Department of Quantum Physics and Astrophysics,

Institute of Cosmos Sciences,

Faculty of Physics, University of Barcelona

c\ Martí i Franquès, 1

08028, Barcelona, Spain

Tel: +34 9340 39396

e-mail: arnau.rios_at_icc.ub.edu

twitter: @riosarnau